From Writers Helping Writers:
Debilitating fears are a problem for everyone, an unfortunate part of the human experience. Whether they’re a result of learned behavior as a child, are related to a mental health condition, or stem from a past wounding event, these fears influence a character’s behaviors, habits, beliefs, and personality traits. The compulsion to avoid what they fear will drive characters away from certain people, events, and situations and hold them back in life.
In your story, this primary fear (or group of fears) will constantly challenge the goal the character is pursuing, tempting them to retreat, settle, and give up on what they want most. Because this fear must be addressed for them to achieve success, balance, and fulfillment, it plays a pivotal part in both character arc and the overall story.
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Fear of Infidelity
Notes
Relationships are built on trust, and when one partner cannot trust the other, the entire relationship is jeopardized. In some cases, the partner may have a history of cheating or questionable relationships. In other situations, the character may be projecting their own self-doubt and insecurities onto their partner. This fear could become so great in the character’s mind that, despite a desire or need for romantic relationships, they’re avoided altogether.
The character doubting their own abilities and attractiveness
Being overly jealous or territorial
Secretly checking up on a partner—accessing their phone without permission, following them, etc.
Trying to “trick” a partner into confessing to suspected indiscretions
Enlisting friends to spy on the other party
The character overcompensating to impress their partner
Worrying excessively if a partner is late or doesn’t call
Checking in obsessively via texts or phone calls
Making unfounded accusations about a lover’s faithfulness
Being overly needy
Forbidding a partner to have relationships that aren’t sanctioned by the character
“Catfishing” a significant other online (or having a friend do it)
Bending over backwards to please a lover
The character agreeing to bedroom activities they’re not comfortable with to appease their partner
Common Internal Struggles
The character wanting to trust their partner but being unable to do so
The character worrying that the partner is disappointed in them (physically, intellectually, etc.)
Experiencing soaring anxiety despite having no tangible reason for it
The character questioning their suspicions (Is this real or am I being paranoid?)
Feeling guilty about spying on or checking up on a partner
The character wanting to discuss their suspicions but also being afraid to find out the truth
Link to the rest at Writers Helping Writers